Geoscience Reference
In-Depth Information
Ocean acidification
Direct measurements of the ocean's chemistry have shown that it has become more acidic
(see Figure 26 ), which is shown by its lower pH. This is because carbon dioxide in the at-
mosphere dissolves in the water of the surface ocean. This is controlled by two main
factors: the amount of carbon dioxide in the atmosphere and the temperature of the ocean.
The oceans have already absorbed about a third of the carbon dioxide resulting from human
activities, which has lead to a steady decrease in ocean pH levels. With increasing atmo-
spheric carbon dioxide in the future the amount of dissolved carbon dioxide in the ocean
will continue to increase. Some marine organisms, such as corals, foraminifera, coccoliths,
and shellfish, have shells composed of calcium carbonate which dissolves more readily in
acid. Laboratory and field experiments show that under high carbon dioxide the more acid-
ic waters cause some marine species to develop misshapen shells and have lower growth
rates, although the effect varies among species. Acidification also alters the cycling of nu-
trients and many other elements and compounds in the ocean, and it is likely to shift the
competitive advantage among species, and have impacts on marine ecosystems and the
food web. This is a major worry as fishing is still a major source food, with about 95 milli-
on tonnes caught by commercial fishing and another 50 million tonnes produced by fish
farms per year.
26. Ocean acidification
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